Secure and efficient nuclear reactor is one of the key technologies to solve the energy resource shortage problems. Since the pebble-bed high temperature gas cooled reactor has a high power generation efficiency and a good inherent safety, and the fuel can be loaded and unloaded without turning off the reactor, it is very popular in many countries.
Presently, the principle of a fuel component detecting device applied in the pebble-bed reactor is detecting the fuel component based on effects of a graphite conductor on the inductive reactance of the coil, and the detecting device can be divided into the following categories according to different structures. The first category is the inboard detecting device in which the detecting coil is predisposed in the stainless steel fuel ball pipeline and when the fuel ball passes through the detecting coil, the inductive reactance of the coil changes, and a fuel ball signal can be obtained by detecting the inductive reactance changes of the coil. The second category is the detecting device installed via side wall drillings in which a hole is formed in the side wall of the stainless steel fuel ball pipeline and the detecting coil is disposed in the hole. The second category device is similar to the electromagnetic proximity switch.
Since the installation of the above two devices both may influence the gas tightness of the pipeline, the sensor structure and the installation have to ensure a gas tightness of the ball pipeline under a high pressure, resulting in the structure complication and mounting difficulty of the sensor. When a fault occurs in the sensor, in order to maintain and replace the sensor, the fuel ball pipeline is needed to be dismantled, which has a long construction time and easily causes radiation pollution, thus influencing utilizability of the reactor. Furthermore, the detecting coil, the frames and other attachments contact with the radioactivity fuel ball directly, which affects a working life of the sensor, so the anti-radiation capability of the sensor material is required to be good.
In addition, the high temperature gas cooled reactor has various electromagnetic interferences and heavy nuclear radiation, and the working environment of the reactor is so complex that it's difficult to check and maintain the devices. The conventional detecting devices have the following defects: the fuel ball detecting device has a low electromagnetic compatibility and is easy to be interfered by an electromagnetic environment in the high temperature gas cooled reactor, thus resulting in a miscount; when a fault occurs in the fuel ball detecting device, the detecting device cannot generate an alarm initiatively, such that a counting loss and a miscount may occur during the failure period.